The present invention relates to scanning devices and, more particularly, to the measurement and correction of a scan line length error, where the scan line is produced from a facet of a rotating reflector.
Printing devices such as laser printers, digital photocopiers and fax machines use a laser beam to write an image on a photosensitive surface. The surface moves and the laser beam scans an image by sweeping in a direction perpendicular to that of the moving surface. This scanning action is similar to the movement of an electron beam across a television tube or other cathode ray tube (CRT). However, unlike a CRT, one dimension of the image, call it the Y dimension, is controlled by the movement of the surface, while the other dimension, call it the X dimension, is controlled by the scanning action of the laser.
Laser imaging devices implement the scanning action by reflecting a laser beam off a rotating reflective surface, typically a rotating mirror. The rotating mirror is a polygon typically having two or more faces called facets that reflect the laser beam. Mirrors with four, six or eight facets are not uncommon.
In a laser imaging system having a mirror with a plurality of facets, the quality of an image is affected by several factors related to the design and manufacture of the facets. One such factor is a radial runout of a polygon mirror after it is attached to a shaft of a drive motor. Radial runout is a total variation in a direction perpendicular to an axis of rotation of a reference surface from a surface of revolution. In this case, the reference surface is the motor shaft. Radial runout includes errors due to eccentricity and out-of-roundness.
An ideal system operates within the constraints listed below.
(1) In the case of an even-sided polygonal mirror, facets on opposite sides of the mirror are parallel to one another.
(2) The mirror has minimal runout relative to its rotational center.
(3) The angles formed by the facets of the polygon are precise.
(4) The motor and bearing system run true, without wobble.
A system that fails to meet these constraints can produce objectionable artifacts in a printed image. These artifacts are due to scan lines of different lengths.
In a system using a multi-facet mirror, successive facets of the mirror produce successive scan lines of the image. Thus, a specific facet of the rotating mirror produces specific scan lines. For example, a four-faceted mirror will produce scan lines as shown in Table 1, below.
Imperfections in the mirror facets can cause scan lines to be of different lengths. For example, all the scan lines written by a facet can be one length, while those written by another facet are a different length. Presently, scanners can produce scan lines with 300 or 600 dots per inch (DPI) that vary less than one dot per line. Nonetheless, even a minor difference in the length of a scan line can cause a periodic distortion in an image.
The beginning of each scan line is electronically synchronized to a starting margin of an image. The synchronizing signal is conventionally known as a xe2x80x9cbeam detectxe2x80x9d (BD). A variation in the scan line accumulates over the length of the scan line and typically reaches its maximum at the end of the scan line.
An observer will usually not notice any variation in a single line. However, a periodic pattern produced by the variation in the scan line may interfere with a pattern of gray scale or halftones in an image, thus creating a moirxc3xa9 pattern. A moirxc3xa9 pattern typically appears as a periodic series of lines superimposed over the image. Even though differences in the lengths of the scan lines are less than one dot wide, the human vision system is very sensitive to moirxc3xa9 patterns and it can detect errors of less than the width of one dot. For example, a human can detect a moirxc3xa9 pattern caused by a xc2xc dot error in an image produced by a 600 DPI printer. As laser printers are called upon to print images approaching photograph quality, gray scale and halftone patterns are used more frequently, and the resulting images are more susceptible to moirxc3xa9.
Methods exist that compensate for scanner imperfections by lengthening or shortening the lines produced by the facets until each line is the same length. A system applying such a method requires knowledge of (1) the amount of facet-to-facet imperfection, and (2) which facet of the mirror is reflecting the laser beam.
Knowledge of the amount of facet-to-facet imperfection is used to determine how much compensation is required for a particular facet. A facet error can be characterized in terms of the time it takes to sweep a beam across a predetermined length. For example, a 600 DPI printer that prints a line across an eight-inch page prints 4800 dots.
4800 dot=600 dots/inchxc3x978 inches
A particular model of printer may print a dot in 50 nanoseconds (ns). Thus an eight-inch line would be printed in 240 microseconds (xcexcs).
240 xcexcs=4800 dotsxc3x9750 ns/dot
If the nominal scan line is 240 xcexcs long, then a scanner imperfection that causes a scan line length of 240.050 xcexcs corresponds to a length of one extra dot. A scan line length of 239.950 corresponds to a line that is one dot shorter than the nominal line. As humans can detect moirxc3xa9 patterns caused by a xc2xc dot error, a facet-to-facet deviation of 12.5 ns can result in a noticeable imaging artifact.
Knowledge of which facet of the mirror is reflecting the laser beam is necessary so that an appropriate compensation can be applied when a particular facet is producing a scan line. The scan line lengths are corrected on a facet-by-facet basis so that all the resulting printed lines on the page are the same lengths. The correction for a scan line is achieved by shifting the time at which a pixel is printed, thus resulting in a shift of the point at which the pixel is printed so that adjacent raster lines are aligned with one another. For example, a pixel at row 5, column 400 will properly align with a pixel at row 6, column 400.
One technique for characterizing a scanner involves the use of a test fixture to measure and record the scan line length of each facet. This information is either physically written onto the scanner, or stored into an electronic memory that is included with the scanner. The information is subsequently recalled during a line length correction procedure. Because the measurement is made external to the scanner system, this technique requires additional manufacturing steps for the characterization process, and further requires a step for a transfer of the characterization information from the scanner to a compensation circuit. Both steps add to the cost of the scanner in terms of handling in order to perform the measurement, as well as in hardware such as cables, connectors and memory circuitry for storage of the measured values.
Systems exist in which a scan line length is measured in real-time as a scan line is produced from a facet. A first sensor is located to detect a beginning of the scan line, and a second sensor is located to detect the end of the scan line. The scan line length is the interval of time that elapses between detection of the beginning and the end of the scan line. Such a system includes circuitry and wiring to support the two sensors. Conventionally, a line length correction is thereafter based on the difference between the actual length of the line and a desired, ideal length of the line without considering the lengths of the scan lines relative to one another. A system performing real-time measurement would benefit from an improvement that reduces circuitry and/or improves resolution. Also, if the correction were made by adjusting the line lengths relative to a line length of a reference facet, rather than to a desired, ideal length, then at least one less correction would need to be made since no correction would be required for the reference facet.
An existing technique for identifying which facet of a mirror is reflecting the laser beam is to tag one facet so that a sensor can detect the tagged facet. For example, the tag might be a physical mark that is sensed optically. Assume that facet #1 is tagged. The sensor will detect facet #1, and thereafter a beam detect circuit counts subsequent facets until the mirror makes a full rotation bringing facet #1 into printing position again. Disadvantageously, this technique requires a means for tagging a facet, a sensor for detecting the tag and wiring to communicate the facet information to the compensation circuitry. Also, when a system uses a counter to keep track of which facet is producing a scan line, the count must cycle based on the number of facets on, or the modulo number of, the polygon mirror. Conventional systems are therefore limited to operating with a scanning device having a particular, predetermined number of facets.
Accordingly, there is a need for a scanning device of reduced cost and complexity that can detect both a beginning and an end of a scan line with a single detector.
There is also a need for an improved method for determining a scan line length correction for each of the facets of a polygon mirror.
Furthermore, there is a need for a method for determining a modulo number that indicates a number of facets on a polygon mirror in a scanning device.
A first method of the present invention determines a scan line error for a scan line, wherein the scan line is produced from one of a plurality of facets of a rotating reflector of a scanning device. The method comprises the steps of (a) determining a difference between a time of an occurrence of a point in a scan line produced from a first facet and a time of an occurrence of a point in a scan line produced from a second facet, and (b) determining from the difference, a scan line error for the scan line produced from the first facet.
A second method of the present invention determines a correction for a scan line, wherein the scan line is produced from one of a plurality of facets on a rotating reflector of a scanning device. The method comprises the steps of (a) determining a facet error from a difference between a length of a scan line produced from a first facet and a length of a scan line produced from a second facet, and (b) determining from the facet error, a correction for the scan line produced from the first facet.
A third method of the present invention determines a modulo number that indicates a number of facets on a rotating polygon mirror of a scanning device. The polygon mirror has a plurality of facets. The method comprises the steps of: (a) acquiring N consecutive scan line lengths produced from the plurality of facets, where N is an integer substantially greater than an expected value of the modulo number, (b) calculating an average scan line length from the N scan line lengths, (c) calculating a set of error values for the N scan line lengths, through use of each of the N scan line lengths and the average scan line length, (d) determining from the set of error values an index X to a peak error value, and an index Y to a subsequent occurrence of a further peak error value, and (e) determining the modulo number from a difference between Y and X. A system is also provided to perform this method.
An embodiment of the present invention provides for an apparatus for detecting a first point and a second point of a scan line that is produced by a light beam reflected from a facet of rotating reflector. The light beam travels from the facet along a first path to the first point, and the light beam travels from the facet along a second path to the second point. The apparatus comprises: (a) a detector located in the first path for detecting the light beam, and (b) a mirror located in the second path for reflecting the light beam toward the detector. The detector produces a signal when the light beam is incident at the first point or incident at the second point.